Sport activities such as jogging, biking, diving etc. are often practiced with a device that display sport related data, such as heart pulse, speed, pace, calories burnt, and so on. These sport-related data can be displayed with a head-mounted display, for example glasses. In particular, sport data can be displayed with a retinal projection device.
Retinal projection devices are known as such. They usually comprise a light source and a MEMS-based mirror arranged for orienting the laser beam directly onto a selected portion of the retina of the user wearing the device. By scanning the laser beam and controlling its intensity, the device projects an image that is superimposed over the image of the scene viewed by the user.
Conventional retinal displays are usually conceived so as to replace a large portion of user's field of view by a projected image. In this case, the user does not see the scene behind the projected area, which is unacceptable for many sport applications where the sportsman needs a largely unobstructed field of view.
Retinal displays that display a large transparent image overlaid over the real scene are also known. In this case, the user is able to see simultaneously the scene of the real word with an image or some objects superimposed over this view. This is also very distracting for most sport activities where the sportsman needs to concentrate most of the time exclusively on the real scene, and only occasionally wants to see data related to his sport activity.
Displaying a large image with a retinal display requires fast moving mirrors and fast transmission rates for transmitting large image frames. This results in a high power consumption, and in voluminous batteries that frequently needs to be replaced or recharged.
Most retinal display systems moreover require dedicated glasses. Those glasses are usually not well suited for sport activities. Different sports require different type of glasses. For example, large glasses with optimal sun and fog protection are often used for skiing, while jogging is preferably made with lighter sun glasses. Users who need medical glasses have difficulties in using conventional retinal display systems.
Some of these retinal display systems project an image which has fixed optical characteristics such as brightness, luminance, contrast, etc., thus resulting in difficulties for the user when looking at the image in different ambient conditions.
In addition to the above, it is worth pointing out that if the display of the retinal projection device is always active during the usage of the retinal projection device itself (the usage of the glasses), this can lead to high power consume of the batteries of the glasses and can be disturbing for the user, producing unnecessary exposure or fatigue for him.
The user focus is on the sport activity, in particular for demanding ones such as interval training for a runner, while prompting of relevant information are needed punctually or by the user request. Outside of these triggers, the display should remain off, to enhance the user focus on the activity (no distraction), as well as his comfort of vision (no disturbing glowing display light). On the other hand, when valuable activity-related data are needed by the user, these should be obtained rapidly, in an easy (natural) and hassle-free (non distracting) way.
Accordingly, there is a need to turn on and off the projector display on demand in a rapid, natural, non-distracting way, thus also improving the safety of the user himself as unwanted distraction could lead to injuries.
There is also a need to adapt automatically and dynamically the luminance of the projected image as a function of the ambient light conditions which can vary greatly during glasses usage. The goal is to firstly increase the user comfort of vision by optimizing luminance and secondly to efficiently manage the light source and therefore optimize the system power consumption.
In addition there is a need to save current consumption of the battery powered glasses when the displayed information is not needed. The extension of time between two charges, i.e. the autonomy of the glasses, is a key requirement and feature of such glasses. One indeed needs at least to be able to finish a several hours sport exercise or competition without running out of batteries (e.g. a marathon is 3 . . . 5 hours).